The fetus lives in an environment that is exceptionally theromstable but special situations may arise that challenge fetal thermoregulatory capability. These include the hyperthermia of maternal fever and exercise, the hypothermia of extreme cold exposure, and the variable, colder temperatures after birth, when temperature regulation may become critical for survival of the newborn. The mechanisms regulating fetal heat production and elimination in-utero remain largely unknown. We, therefore, propose to continue our studies along three lines of investigation to better understand this aspect of physiology. We will 1) complete the study of fetal heat production using differential calorimetry and correlate its results with oxygen consumption; 2) measure the fraction of heat the fetus loses to the mother through various excretory pathways including the placenta, skin, cord, amniotic fluid, and uterine wall; and 3) explore the various thermoregulatory responses the fetus and newborn make to maintain a constant temperature. These responses include changes in heat production, heat conductance, and umbilical flow that might stabilize temperature. Investigations will be performed at various times during development to study maturation of responses. Experiments are designed to give new understanding of fetal heat production and temperature control, and how these change throughout gestation and to assess their physiologic importance.